Attribute Groups

In the EclipseLink 2.1.0 release, EclipseLink introduced the concept of an AttributeGroup that can be used to configure the use of partial entities in fetch, load, copy, and merge operations. This example illustrates how this functionality can be used.

Fetch: Control which attributes and their associated columns are retrieved from the database

Load: Control which relationships in the entities returned from a query are populated

Copy: Control which attributes are copied into a new entity instance

Merge: Merge only those attributes fetched, loaded, or copied into an entity

AttributeGroup Types and Operations

FetchGroup

The FetchGroup which has existed in EclipseLInk for a number of releases defines which attributes should be fetched (selected from the database) when the entity is retrieved as the result of a query execution. The inclusion of relationship attributes in a FetchGroup only determines if the attribute's required columns should be fetched and populated. In the case of a lazy fetch type the inclusion of the attribute simply means that its proxy will be created to enable lazy loading when accessed. To force a relationship mapping to be populated when using a FetchGroup on a query the attribute must be included in the group and must either be FetchType.EAGER or it must be included in an associated LoadGroup on the query.

Default FetchGroup

FetchGroup also has the notion of named and default FetchGroup which are managed by the FetchGroupManager. A default FetchGroup is defined during metadata processing if one or more basic mappings are configured to be lazy and the entity class implements FetchGroupTracker (typically introduced through weaving). The default FetchGroup is used on all queries for this entity type where no explicit FetchGroup or named FetchGroup is configured.

Named FetchGroup

A Named FetchGroup can be defined for an entity using [TODO @FetchGroup] or within the [[EclipseLink/Examples/JPA/EclipseLink-ORM.XML|eclipselink-orm.xml]

Full FetchGroup

A FetchGroup when first created is assumed to be empty. The user must add the attributes to the FetchGroup. If a FetchGroup is required with all of the attributes then the FetchGroupManager.createFullFetchGroup() must be used.

Load/LoadAll with FetchGroup

A FetchGroup can also be configured to perform a load operation of relationship mappings and nested relationship mappings.

LoadGroup

A LoadGroup is used to force a specified set of relationship attributes to be populated in a query result.

CopyGroup

The CopyGroup replaces the deprecated ObjectCopyPolicy being used to define how a entity is copied. In addition to specifying the attributes defining what should be copied from the source entity graph into the target copy the CopyGroup also allows definition of:

shouldResetPrimaryKey: Reset the identifier attributes to their default value. This is used when the copy operation is intended to clone the entity in order to make a new entity with similar state to the source. Default is false.

shouldRestVersion: Reset the optimistic version locking attribute to its default value in the copies. Default is false.

depth: defines cascade mode for handling relationships. By default CASCADE_PRIVATE_PARTS is used but it can also be configured to NO_CASCADE and CASCADE_ALL_PARTS.

a new depth () not available in ObjectCopyPolicy) CASCADE_TREE is default in case the group has at least one attribute (addAttribute method has been called on the group).

There are significant differences between behaviours of CopyGroup with CASCADE_PRIVATE_PARTS / CASCADE_ALL_PARTS / NO_CASCADE vs. CASCADE_TREE:

CASCADE_PRIVATE_PARTS / CASCADE_ALL_PARTS / NO_CASCADE: if true then the primary key of the copy is not set;

CASCADE_TREE: if false then the primary key attributes are always copied, if true then only primary key attribute(s) that are not specified in the group are not set (explicitly specified in the group primary key attributes are always copied);

if set to true, then the copy object never assigned a fetch group;

use this option to create a new object from original by persisting the copy (usually after setting some new attribute values);

if set to false, then copy object has a fetch group if there is at least one non copied attribute.

use this (default) option for sparse merge.

shouldResetVersion

CASCADE_PRIVATE_PARTS / CASCADE_ALL_PARTS / NO_CASCADE: ignored;

CASCADE_TREE: if false then version attribute is always copied, if true then copied only if specified in the group (explicitly specified in the group version attribute is always copied);

Merging

When a partial entity is merged into a persistence context that has an AttributeGroup associated with it defining which attributes are available only those attributes are merged. The relationship mappings within the entity are still merged according to their cascade merge settings.

Copy Examples

Here an Employee entity is copied with it basic names, address's city and street and phoneNumbers' area code and number

CopyGroup group =new CopyGroup();// Because the group has attributes CASCADE_ATTRIBUTES is used by default: only copy the specified attribute group items
group.addAttribute("firstName");
group.addAttribute("lastName");
group.addAttribute("address.city");
group.addAttribute("address.street");
group.addAttribute("phoneNumbers.areaCode");
group.addAttribute("phoneNumbers.number");
Employee empCopy =(Employee) em.unwrap(JpaEntityManager.class).copy(emp, group);

If the reference attribute specified as a "leaf" (with no sub attributes) then all its non-reference attributes are copied (but primary key is copied, too - even if it's a reference attribute).
Let's assume that Address class doesn't have any non-reference attributes - then

Let's assume that PhoneNumber class has a composite primary key: {owner, type} - where owner is a reference to it's owner Employee and type is a string (that code have values like 'Home", "Work", "Cell etc).
Just like in Address's case, adding phoneNumber attribute is equivalent to adding all PhoneNumber's attributes (the only reference attribute "owner" is also added because it's part of primary key).
Because of that

group.addAttribute("phoneNumbers");

would trigger copying of the whole owner Employee - not just firstName and lastName we wanted to copy.
To overcome that there is a way to specify that we don't need the whole owner:

Clone Entity using Copy and Persist

In this example an employee entity is cloned using a CopyGroup to copy all attributes with the exception of relationships (1:1, 1:M, M:M) that are not configured as private-owned. The resulting copy of the entity is then persisted to have new identity but the same state as its source.

CopyGroup group =new CopyGroup();// Because the group has no attributes CASCADE_PRIVATE_PARTS depth is used by default: copy all attributes and only private-owned relationships
group.setShouldResetPrimaryKey(true);
Employee empCopy =(Employee) em.unwrap(JpaEntityManager.class).copy(emp, group);System.out.println(">>> Employee copied");// pk should be reset
empCopy.setId(newId);// non-copied attributes are shared with the original - most (or all) of them usually have to be cleared and reset.
empCopy.setAddress(newAddress);
empCopy.setPhoneNumbers().clear();
empCopy.addPhoneNumber(new PhoneNumber(...));
empCopy.addPhoneNumber(new PhoneNumber(...));// Persist the employee copy
em.persist(empCopy);